Re: [PATCH v1 10/14] mm: multigenerational lru: core

[Yu Zhao]

On Tue, Mar 16, 2021 at 04:53:53PM +0800, Huang, Ying wrote:
> Yu Zhao <yuzhao@xxxxxxxxxx> writes:
> 
> > On Tue, Mar 16, 2021 at 02:52:52PM +0800, Huang, Ying wrote:
> >> Yu Zhao <yuzhao@xxxxxxxxxx> writes:
> >> 
> >> > On Tue, Mar 16, 2021 at 10:08:51AM +0800, Huang, Ying wrote:
> >> >> Yu Zhao <yuzhao@xxxxxxxxxx> writes:
> >> >> [snip]
> >> >> 
> >> >> > +/* Main function used by foreground, background and user-triggered aging. */
> >> >> > +static bool walk_mm_list(struct lruvec *lruvec, unsigned long next_seq,
> >> >> > +			 struct scan_control *sc, int swappiness)
> >> >> > +{
> >> >> > +	bool last;
> >> >> > +	struct mm_struct *mm = NULL;
> >> >> > +	int nid = lruvec_pgdat(lruvec)->node_id;
> >> >> > +	struct mem_cgroup *memcg = lruvec_memcg(lruvec);
> >> >> > +	struct lru_gen_mm_list *mm_list = get_mm_list(memcg);
> >> >> > +
> >> >> > +	VM_BUG_ON(next_seq > READ_ONCE(lruvec->evictable.max_seq));
> >> >> > +
> >> >> > +	/*
> >> >> > +	 * For each walk of the mm list of a memcg, we decrement the priority
> >> >> > +	 * of its lruvec. For each walk of memcgs in kswapd, we increment the
> >> >> > +	 * priorities of all lruvecs.
> >> >> > +	 *
> >> >> > +	 * So if this lruvec has a higher priority (smaller value), it means
> >> >> > +	 * other concurrent reclaimers (global or memcg reclaim) have walked
> >> >> > +	 * its mm list. Skip it for this priority to balance the pressure on
> >> >> > +	 * all memcgs.
> >> >> > +	 */
> >> >> > +#ifdef CONFIG_MEMCG
> >> >> > +	if (!mem_cgroup_disabled() && !cgroup_reclaim(sc) &&
> >> >> > +	    sc->priority > atomic_read(&lruvec->evictable.priority))
> >> >> > +		return false;
> >> >> > +#endif
> >> >> > +
> >> >> > +	do {
> >> >> > +		last = get_next_mm(lruvec, next_seq, swappiness, &mm);
> >> >> > +		if (mm)
> >> >> > +			walk_mm(lruvec, mm, swappiness);
> >> >> > +
> >> >> > +		cond_resched();
> >> >> > +	} while (mm);
> >> >> 
> >> >> It appears that we need to scan the whole address space of multiple
> >> >> processes in this loop?
> >> >> 
> >> >> If so, I have some concerns about the duration of the function.  Do you
> >> >> have some number of the distribution of the duration of the function?
> >> >> And may be the number of mm_struct and the number of pages scanned.
> >> >> 
> >> >> In comparison, in the traditional LRU algorithm, for each round, only a
> >> >> small subset of the whole physical memory is scanned.
> >> >
> >> > Reasonable concerns, and insightful too. We are sensitive to direct
> >> > reclaim latency, and we tuned another path carefully so that direct
> >> > reclaims virtually don't hit this path :)
> >> >
> >> > Some numbers from the cover letter first:
> >> >   In addition, direct reclaim latency is reduced by 22% at 99th
> >> >   percentile and the number of refaults is reduced 7%. These metrics are
> >> >   important to phones and laptops as they are correlated to user
> >> >   experience.
> >> >
> >> > And "another path" is the background aging in kswapd:
> >> >   age_active_anon()
> >> >     age_lru_gens()
> >> >       try_walk_mm_list()
> >> >         /* try to spread pages out across spread+1 generations */
> >> >         if (old_and_young[0] >= old_and_young[1] * spread &&
> >> >             min_nr_gens(max_seq, min_seq, swappiness) > max(spread, MIN_NR_GENS))
> >> >                 return;
> >> >
> >> >         walk_mm_list(lruvec, max_seq, sc, swappiness);
> >> >
> >> > By default, spread = 2, which makes kswapd slight more aggressive
> >> > than direct reclaim for our use cases. This can be entirely disabled
> >> > by setting spread to 0, for worloads that don't care about direct
> >> > reclaim latency, or larger values, they are more sensitive than
> >> > ours.
> >> 
> >> OK, I see.  That can avoid the long latency in direct reclaim path.
> >> 
> >> > It's worth noting that walk_mm_list() is multithreaded -- reclaiming
> >> > threads can work on different mm_structs on the same list
> >> > concurrently. We do occasionally see this function in direct reclaims,
> >> > on over-overcommitted systems, i.e., kswapd CPU usage is 100%. Under
> >> > the same condition, we saw the current page reclaim live locked and
> >> > triggered hardware watchdog timeouts (our hardware watchdog is set to
> >> > 2 hours) many times.
> >> 
> >> Just to confirm, in the current page reclaim, kswapd will keep running
> >> until watchdog?  This is avoided in your algorithm mainly via
> >> multi-threading?  Or via direct vs. reversing page table scanning?
> >
> > Well, don't tell me you've seen the problem :) Let me explain one
> > subtle difference in how the aging works between the current page
> > reclaim and this series, and point you to the code.
> >
> > In the current page reclaim, we can't scan a page via the rmap without
> > isolating the page first. So the aging basically isolates a batch of
> > pages from a lru list, walks the rmap for each of the pages, and puts
> > active ones back to the list.
> >
> > In this series, aging walks page tables to update the generation
> > numbers of active pages without isolating them. The isolation is the
> > subtle difference: it's not a problem when there are few threads, but
> > it causes live locks when hundreds of threads running the aging and
> > hit the following in shrink_inactive_list():
> >
> > 	while (unlikely(too_many_isolated(pgdat, file, sc))) {
> > 		if (stalled)
> > 			return 0;
> >
> > 		/* wait a bit for the reclaimer. */
> > 		msleep(100);
> > 		stalled = true;
> >
> > 		/* We are about to die and free our memory. Return now. */
> > 		if (fatal_signal_pending(current))
> > 			return SWAP_CLUSTER_MAX;
> > 	}
> >
> > Thanks to Michal who has improved it considerably by commit
> > db73ee0d4637 ("mm, vmscan: do not loop on too_many_isolated for
> > ever"). But we still occasionally see live locks on over-overcommitted
> > machines. Reclaiming threads step on each other while interleaving
> > between the msleep() and the aging, on 100+ CPUs.
> 
> Got it!  Thanks a lot for detailed explanation!

You are always welcome. Just a side note as you and Dave are working
on migrating pages from DRAM to AEP: we also observed migrations can
interfere (block) reclaims due to the same piece of code above. It
happened when there were a lot of compaction activities going on. My
guess is it could happen to your use case too. Migrations can isolate
a large number of pages, see migrate_pages().